Ian Deary and Geoff Der measured the IQ and reaction times of 898 people (average age 55 years) in 1988 and then noted which of them had died by 2002 (185 had). They also recorded information on each person's education, occupation and whether or not they smoked.

In 1988, being male, a smoker, and of lower social class were all independent significant predictors of the likelihood of dying before 2002. But even when these factors were controlled for, having lower IQ and slower reactions were also significant predictors of the likelihood of dying, roughly equivalent to the difference made by whether one smoked or not. Reaction time seemed to be the crucial factor here - IQ and reactions correlated, and when reactions were controlled for, IQ no longer predicted the likelihood of dying.

But in our modern world of settees and coffee shops, why should having faster reactions increase your life expectancy? "These variables might...constitute a simple... indicator of the organism's integrity, and of clinical and subclinical pathology" the authors said.

But what about a study started in 1932 that showed 11-year-old children's IQ predicted their mortality 70 years later? Surely IQ wasn't revealing latent pathology at that age? Perhaps, the authors said, "...even at that age lower IQ relates to earlier death partly because it is a reflection of a body with suboptimal physical integrity".
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